Thermoplastic resin films such as polyamide film and polyester film are excellent in strength, transparency and formability, and accordingly are used as packaging materials in a wide range of applications. However, these thermoplastic resin films are high in the permeability for a gas such as oxygen, and hence when these thermoplastic resin films are used for packaging general food, retort processed food, cosmetics, medical supplies, agricultural chemicals and the like, long-term storage of such packaged objects may cause the deterioration of the contents of the packages due to the gas such as oxygen penetrating through the film.
Thus, multilayer films in which a polyvinylidene chloride (hereinafter abbreviated as “PVDC”) layer having a high gas-barrier property is formed by coating the surface of a thermoplastic resin with an emulsion of PVDC or the like are widely used for food packaging and the like. However, PVDC generates organic substances such as acidic gases at the time of incineration of packaging films after the disposal of the packaging films. Accordingly, in these years, switching to other materials has been strongly desired as the concern about the environment has grown.
As a substitute material for PVDC, polyvinyl alcohol (hereinafter abbreviated as “PVA”) does not generate poisonous gas and is high in the gas-barrier property under low-humidity atmosphere. However, the gas-barrier property of PVA is steeply degraded with increasing humidities. Accordingly, in many cases, it is impossible to use PVA for packaging water-containing food or the like.
Known as a polymer improved in such gas-barrier degradation as found for PVA under high humidity is a copolymer between vinyl alcohol and ethylene. Hereinafter the copolymer is abbreviated as “EVOH”. However, for the purpose of maintaining at a practical level the gas-barrier property under high humidity, it is necessary to increase the copolymerization proportion of ethylene to some extent, and such a polymer as obtained with increased ethylene proportion becomes poorly soluble in water. Thus, for the purpose of obtaining a coating material by using an EVOH that is high in the copolymerization proportion of ethylene, it is necessary to use an organic solvent or a mixed solvent composed of water and an organic solvent. However, such a case is not desirable from the viewpoint of environmental issues, due to the use of an organic solvent, and the cost increase is disadvantageously caused by the necessity of a step of collecting the organic solvent or other steps.
As methods in which a liquid composition composed of a water-soluble polymer is applied to a film to attain a high gas-barrier property even under high humidity, there have been proposed methods in which an aqueous solution composed of PVA and a partially neutralized product of polyacrylic acid or polymethacrylic acid is applied to a film and heat treated for cross-linking of both polymers through ester bonds (JP-A-06-220221, JP-A-07-102083, JP-A-07-205379, JP-A-07-266441, JP-A-08-041218 and JP-A-10-237180).
However, the methods proposed in these documents require, for the purpose of attaining a high gas-barrier property, a heat treatment at high temperatures or a heat treatment over a long period of time, and thus require a huge amount of energy at the time of production of a gas-barrier layer to result in loading not a small amount of burden on the environment.
Additionally, such a heat treatment at high temperatures causes a possibility that PVA or the like constituting the gas-barrier layer is discolored or decomposed, and moreover, the substrate such as a plastic film having the gas-barrier layer laminated thereon undergoes deformation such as wrinkle formation so as to inhibit the use as a packaging material. For the purpose of preventing the deterioration of the plastic substrate, it is required to use as the substrate a specific heat-resistant plastic film sufficiently resistible to high-temperature heating, accordingly leading to a drawback from the viewpoints of general versatility and economic efficiency.
On the other hand, when the heat treatment temperature is low, an extremely long-time treatment is required and hence the productivity is disadvantageously degraded.
An investigation has also been made to solve the problem found in the above-described film coated with PVA by introducing a cross-linked structure into PVA. However, in general, although the humidity dependence of the oxygen gas-barrier property of the PVA film is decreased with the increase of the cross-link density, the oxygen gas-barrier property intrinsically possessed by the PVA film under dry conditions is degraded. Consequently, it is extremely difficult to obtain a satisfactory oxygen gas-barrier property under high humidity.
In general, although cross-linking of polymer molecules improves the water resistance, because the gas-barrier property is a property associated with the prevention of penetration or diffusion of relatively small molecules such as oxygen molecules, simple cross-linking of a polymer does not necessarily lead to attaining gas-barrier property. For example, three-dimensionally cross-linked polymers of epoxy resin, phenolic resin and the like have no gas-barrier property.
There is proposed a method in which a gas-barrier multilayer body that has a high gas-barrier property, despite the use of a water-soluble polymer such as PVA, even under high humidity is obtained with a heat treatment lower in temperature or shorter in time than in conventional cases (JP-A-2001-323204, JP-A-2002-020677 and JP-A-2002-241671). The coating agents described in these documents enable to form gas-barrier multilayer bodies having a higher gas-barrier property, despite the use of water-soluble polymers, than in conventional cases even under high humidity, with heating lower in temperature or shorter in time than in the cases of the above-described cross-linking coating agents.
However, when applied are only the methods described in these documents in which methods the hydroxyl groups in PVA and the COOH groups in polyacrylic acid or in an ethylene-maleic acid copolymer are subjected to an esterification reaction with each other by heating, long-time storage of the thus obtained films with a gas-barrier layer formed thereon, under high temperature and high humidity results in the decomposition of the ester bonds due to hydrolysis to remarkably degrade the gas-barrier property.
As described above, only the heating and curing of a coating agent cannot meet more severe requirements.
As a method for producing by applying a moderate heat treatment a gas-barrier film that attains a high-degree gas-barrier property, there have been proposed methods for producing a gas-barrier multilayer body in which methods a coating material for forming a gas-barrier layer composed of a polyalcohol polymer and a polycarboxylic acid polymer is applied, then a short-time heat treatment is conducted, and then a heat treatment is conducted in the presence of water containing a monovalent metal compound and/or a basic organic compound and a divalent or higher-valent metal compound (JP-A-2005-270907, JP-A-2005-271516 and JP-A-2005-272758). According to these production methods, no degradation of the gas-barrier property is found over a long term under high temperature and high humidity atmosphere. However, the above-described methods need a process in which a set of coating and drying or a set of coating, immersing and drying is repeated a plurality of times, and hence are complicated in the production process and may also need a huge amount of energy.
There have been proposed those methods in which by one-time coating with a coating liquid using a polycarboxylic acid and a multivalent metal compound, an ionic cross-linked structure is introduced into a coating layer to attain a high-degree gas-barrier property (JP-A-2005-126528 and JP-A-2005-126539). Also, according to these production methods, no degradation of the gas-barrier property is found over a long term under high temperature and high humidity atmosphere. However, these methods use a volatile base or an organic solvent in a large amount, and hence are not desirable from the viewpoint of environmental issues. Additionally, these methods need a step of collecting the volatile base or the organic solvent and so on, and hence the cost is disadvantageously increased.